
Volume,
Density and Specific Gravity As the pressure on a fluid changes its volume will
also change. For a gas, the change in volume may be large whereas for most liquids
it will be small. The gas is said to be a compressible material and the
liquid an incompressible material. Volume, V, is a parameter that depends
on the size of the system, and in the case of the earth's atmosphere can be very
large. It is, therefore, frequently convenient to identify the volume per
unit mass of a material as its "specific volume:" v = (V/m). The specific volume
is the reciprocal of the density of the material, which is the mass per unit
volume: r = (1/v) = (m/V).
The "Specific
Gravity" of a substance is the ratio of its mass to that of an
equal volume of water at the same temperature and pressure. This is also the
ratio of the densities of the two substances: SG = (mass of a volume V of a material)/(mass
of a volume V of water) = (r_{Material}/r_{Water}).
Two
fluids that will be of interest in humanpowered transportation are water
and air. In the metric system a volume of water was used to define the unit
of mass. At 4 C water has its maximum density of 1000 kg/m^{3}. As
the temperature is increased the thermal expansion of the liquid increases the
volume of this mass of material and the density decreases. At 20 C (room temperature)
the density is 998.2 kg/m^{3}.
Air at atmospheric
pressure (1.013 x 10^{5} Pa.) and 20 C has a density of 1.204 kg/m^{3}.
The specific Gravity of air is the ratio of the densities of air
and water:
SG(Air)
= (r_{Air}/r_{Water}) = (1.024/998.2) = 1.03
x 10^{3}.

